A visually elicited escape response in the fly that does not use the giant fiber pathway.

A housefly elicits an escape in response to an approaching target (Holmqvist & Srinivasan, 1991). This study tests if the giant fiber pathway, which mediates a light-off escape response in a fruitfly (Wyman et al., 1985), also mediates escape to an approaching target in a housefly. Visual stimuli simulating an approaching or receding dark disk were presented to houseflies, Musca domestica, in both behavioral and physiological experiments. Freely behaving flies escaped in response to an expanding dark disk but not to a contracting dark disk. In restrained flies, the giant fiber, here called the giant descending neuron (GDN), was recorded from intracellularly and the tergotrochanteral muscle (TTM), which provides the main thrust in an escape jump, was recorded from extracellularly. During electrical stimulation of the brain, by means of stimulating electrodes inserted into the ventral part of each compound eye, a single spike in the GDN drives the TTM. However, when the TTM responds to visual stimulation that elicits an escape response in a behaving fly, the GDN shows no activity. Similarly to the behavioral results, the TTM of restrained flies showed muscle potentials in response to an expanding dark disk, but not to a contracting disk. However, freely moving flies elicit escapes more than 100 ms on average before the first TTM spike, suggesting that this type of escape does not start with a jump powered by the TTM. In conclusion, this visually evoked escape response in the housefly is not likely to be mediated by the giant fiber pathway. The findings suggest that there exist at least two pathways mediating visually evoked escape responses in flies.